Submitting your site to a directory means it "usually" is reviewed by a human for relevance and quality then categorized based on sites content. One of benefits from a users point of view is that because of this human intervention, they will almost certainly receive relevant listings to there search query. Your benefit as a Webmaster is for same reason, when someone clicks through from a directory to your site you can be certain (if categorized properly in database) that your site contains information that person is looking for.
A few directories to look for submission include:
Yahoo! - Yahoo! charges a fee of $299 for commercial sites to begin evaluation process. This in no way guarantees that your site would be listed in their directory. Yahoo! Directories can be an important source of qualified traffic if listed properly as they do hold one of most popular directories you could ever be listed in.
Open Directory Project - Another extremely popular directory. There is no fee to submit your site to dmoz however it can take several months to get listed. Make sure you review their policies on submitting before attempting to submit. Dmoz editors are very thorough and go through thousands of submissions on a monthly basis. Getting listed in Open Directory Project will certainly get you listed in Google.